1. Field of the Invention
The present invention relates to a demodulation apparatus, a demodulation method, and an electronic device. More particularly, the invention relates to a demodulation apparatus, a demodulation method, and an electronic device for resolving the problem of a so-called null state thereby enhancing the accuracy of demodulation independent of the reception environment.
2. Description of the Related Art
Recent years have witnessed widespread acceptance of noncontact IC cards (e.g., FeliCa (registered trademark)). A variety of demodulation methods have been proposed for communication using such IC cards.
Ordinary noncontact IC card reader/writers have experienced the following problem regarding the demodulation of a transmitted signal load-modulated by a noncontact IC card: upon load modulation, the so-called null state can occur if there is a particular distance between the noncontact IC card and the reader/writer. The null state is a state where communication is not established due to the inability to demodulate the received signal. This state is brought about when there is no amount of change in the direction of amplitude whereas there exists an amount of change in the phase direction.
One proposed method of countering the problem above involves supplementing an ordinary amplitude change detector with a phase change detection circuit so that one of the outputs from these two types of circuits may be selected (e.g., see Japanese Patent Laid-Open No. 2009-118070; called Patent Document 1 hereunder). In determining the output to be selected, the technique proposed by Patent Document 1 references a transmission channel code (sync code) and the result of an average amplitude level having been calculated.
More specifically, digital demodulators described in Patent Document 1 each include a transmission channel code detection circuit and an average amplitude level calculation circuit in addition to circuits for performing demodulation and bit decoding. Detection signals coming from the transmission channel code detection circuit and the average amplitude level calculation circuit are used by a controller in selecting one of output signals from the digital demodulators. The output signal thus selected is fed to an output selector. Based on the selection signal from the controller, the output selector selects and outputs one of the demodulated signals coming from the digital demodulators.
According to Patent Document 1, the selective demodulation process outlined above permits demodulation in the null state.